Sulphur dioxide generators are placed in cartons containing table grapes, the gaseous sulphur dioxide in the carton preventing the growth on the grapes of certain forms of fungi. Where the grapes are to be shipped over long distances it is conventional to use two stage generators. These generate sulphur dioxide relatively rapidly when first placed in the carton. The fast rate of release lasts for a few days and thereafter sulphur dioxide is released at a much slower rate over a period of several weeks.
Various forms of two stage generator are available commercially. The following four types are known to Applicant:    Type 1. This generator is in two parts. The first part consists of a sheet of a material such as Kraft paper which is pervious to water vapour and sulphur dioxide gas. One face of the sheet has thereon a coating comprising a binder which has dispersed therein a substance, or mixture of substances, which generates sulphur dioxide in the presence of water. The second part comprises two sheets of Kraft paper secured together along a pattern of intersecting lines. The lines and the sheets bound a plurality of pockets. Each pocket has therein, in powder form, the substance(s) which when exposed to water vapour generates sulphur dioxide. The sheets are pervious to both water vapour and sulphur dioxide gas. One first part and one second part are placed in each carton.    Type 2. This comprises three sheets of material. One sheet is of paper, such as Kraft paper, and the other two are of what is called poly coated paper. The coating is of polyethylene. The paper sheet is one of the outside sheets. The sequence of layers is paper, polycoating, paper, polycoating, paper. All three sheets are secured together along a pattern of intersecting lines by heat sealing them together. This forms a series of pockets on each side of the centre polycoated paper sheet, each pocket being bounded by said lines and by two of the sheets. All the pockets contain sulphur dioxide generating substance(s). The paper sheet is more readily penetrated by water vapour than the poly coated sheets. Hence moisture reaches the pockets between the paper sheet and the polycoating of one poly coated sheet before it reaches the pockets between the two poly coated sheets.    Type 3. This generator also comprises three sheets of material, two of them being poly coated and secured together along lines which bound a series of pockets with sulphur dioxide generating substances therein. To this extent the type 3 generator is similar to type 2. However, the third sheet, which is a paper sheet, is laminated to the face of one of the poly coated sheets by a laminating adhesive which has sulphur dioxide generating substance dispersed in it. Water vapour penetrates the paper sheet and reaches the laminating adhesive, and the generating substance dispersed in the laminating adhesive, before it penetrates the poly coated sheets and reaches the pockets between the poly coated sheets.    Type 4. This generator comprises two thin plastic films laminated together using a wax which has sulphur dioxide generating material dispersed therein. The outer face of one of the films carries a coating of sulphur dioxide generating material. The coating is covered by a sheet of non-woven material. The coating provides first stage generation and the generating material between the films provides second stage generation.
The present invention seeks to provide an improved form of two stage sulphur dioxide generator.